You probably could have done a better job of explaining your context – just about anything is "possible." For example, if I was among friends, and critiquing a clumsy acceptance speech we'd just seen on television, I might say, "He ummed that speech to death!" However, I doubt I'd use such informal syntax if I was writing an online review of the Grammy Awards, where I expected hundreds if not thousands of readers, and had aspirations to be regarded as a serious journalist.
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J.R.Dec 31 '12 at 6:15

Some onomatopoeia are accepted as viable words. The few examples you give aren't among those commonly accepted. It would be better to put them in a quote, "She kept saying, "ahh, ohh," all the way down the street." Then again, when writing, you are quite free to use English in whatever way with which your editor will be comfortable.